1. Field of the Invention
The present invention relates to a capacitance detector which detects a small change in capacitance.
2. Description of the Related Art
A capacitance detector which uses a differential detection voltage in order to improve an anti-noise property is known (for example, refer to Japanese Unexamined Patent Application Publication No. 2006-253764). FIG. 7 is a view showing the configuration of a capacitance detection circuit disclosed in Japanese Unexamined Patent Application Publication No. 2006-253764, and FIG. 8 is a timing chart of the capacitance detection circuit.
In the first stage of the capacitance detection circuit shown in FIG. 7, a first capacitor C11 is charged to a voltage Vdd and a second capacitor C12 is grounded by turning on only a switch SW1. Then, by turning off the switch SW1 and turning on a switch SW2, a voltage Vo(+) when the amount of electric charge of the first capacitor C11 is equalized in capacitors C11, C12, and C13 is obtained. A (+) voltage of a differential output is obtained by sampling and holding of the voltage Vo(+) in the capacitor C13.
Then, in the second stage, the first capacitor C11 is grounded and the second capacitor C12 is charged to the voltage Vdd by turning off the switch SW2 and turning on a switch SW3. Then, by turning on a switch SW4, a voltage Vo(−) when the amount of electric charge of the second capacitor C12 is equalized in capacitors C11, C12, and C14 is obtained. A (−) voltage of the differential output is obtained by sampling and holding of the voltage Vo(−) in the capacitor C14.
Finally, a switch SW5 is turned on to transmit (+) and (−) sides of the differential output simultaneously to a differential amplifier AMP. As a result, differential detection voltages are obtained.